This invention has been developed in connection with equipment for heating metal billets, either to raise their temperature to a predetermined point for subsequent processing, or to hold the billets for extended periods (known as heat-soaking) to obtain certain alloy characteristics associated with heat treatment. One of the standard forms of oven structure commonly used for this purpose involves an elongated tunnel-like chamber provided with a group of gas burners or other heat-generating means at least along one portion of the structure. Economical use of fuel dictates that the hot gases (including air) within the primary heating chamber be utilized to either pre-heat the billets prior to entrance into the burner area, or maintain the temperature of the billets after they have left this area. The usual construction of elongated ovens of this type is based upon a sufficient length of the tunnel so that a large portion of the length of it is used as secondary heating chamber by including means for inducing movement of the hot gases from the primary chamber along the full length of the structure. Some sort of conveyor is incorporated to induce movement of the billets through the tunnel. When the oven is used merely to raise the billets to a predetermined temperature, a counterflow arrangement between the gas movement and the direction of movement of the conveyor is used so that the relatively cool billets are exposed to the fuel gases in the secondary heat chamber before the billets are received in the area provided with the primary heating equipment. The heat-soaking procedure is somewhat the reverse of this, in that the billets are admitted first to the primary heating chamber and raised quickly to the desired temperature. They are then carried on into the secondary heating chamber, where the presence of the hot gases from the primary chamber serve to maintain the temperature of the billets for the time desired to obtain the necessary alloy characteristics.
Where large metal billets are processed in considerable quantity, the length of the conventional oven structure becomes a problem both with regard to the cost and also to the floor space requirements. It is also obvious that the standard elongated oven in which part of its length is used as a secondary heating chamber exposes the billets to a temperature gradient as the gases are progressively cooled by transmission of their heat to the billets, and by heat loss through the oven structure. Where the heat-soaking procedure is being used, exposure of the billets to a temperature gradient, rather than to a constant temperature, may present a severe limitation where a precise control is necessary.
Applicant has noted the existance of the following patents that are considered pertinent to this type of oven construction:
Kennedy: U.S. Pat. No. 2,707,629, 1955 PA0 Jenkins, Sr.: U.S. Pat. No. 1,615,627, 1927 PA0 Adams, Jr.: U.S. Pat. No. 2,013,905, 1935 PA0 Spain: U.S. Pat. No. 828,865, 1906 PA0 Larsson: U.S. Pat. No. 1,763,624, 1930 PA0 Johnson, et. al.: U.S. Pat. No. 2,762,618, 1956 PA0 Heinemann: U.S. Pat. No. 3,319,349, 1967